Total nitrogen in soil is commonly measured by combusting a soil sample in a specialized analyzer that detects the nitrogen released.
Understanding Total Nitrogen Measurement
Measuring total nitrogen in soil is crucial for assessing soil health, fertility, and nutrient cycling. It includes all forms of nitrogen present in the soil, both organic and inorganic. A standard and precise method involves using analytical instruments like a CHN analyzer.
The Combustion Method
Based on the provided information, a widely used technique for measuring total nitrogen (and carbon) in soil involves high-temperature combustion. Here's a breakdown of the process:
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Sample Preparation:
- Soil samples are first collected and dried.
- They are then finely ground to ensure homogeneity and optimal combustion.
- Subsamples of the dried, finely ground soil are accurately weighed into small foil capsules. This precise weighing is critical for calculating the final percentage.
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Combustion:
- The weighed soil sample in the capsule is introduced into an automated CHN analyzer.
- Inside the analyzer, the sample undergoes high-temperature combustion. This process converts the carbon in the soil into carbon dioxide (CO₂) and the nitrogen into nitrogen gas (N₂).
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Gas Analysis:
- The gases released during combustion (CO₂, N₂, and often H₂O) are separated.
- This separation is typically done using gas chromatography within the analyzer.
- Detectors measure the amount of CO₂ and N₂ produced.
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Calculation:
- Based on the known weight of the original soil subsample and the measured amount of N₂ gas released, the analyzer calculates the total nitrogen content.
- Final values are expressed as the percentage of nitrogen in the dry soil sample. This gives a standardized measure regardless of initial soil moisture.
Summary Steps
Here's a simple outline of the process:
- Prepare Sample: Dry and grind soil, weigh subsample.
- Combust: Heat sample in CHN analyzer.
- Analyze Gases: Measure released N₂ via gas chromatography.
- Calculate: Determine percentage of N in dry soil.
This method, utilizing techniques like automated combustion and gas chromatography in a CHN analyzer, provides a highly accurate and reproducible way to quantify the total nitrogen content in soil samples.
